1. Field of the Invention
The present invention relates to a low silica X-type zeolite binderless shaped product which is a shaped product with a low SiO2/Al2O3 molar ratio and a low X-type zeolite binder content. More particularly, it relates to a high purity, low silica X-type zeolite binderless shaped product which has a high purity and notably high adsorption capacity, which has excellent mechanical strength, and which is suitable for purposes such as separation and concentration of oxygen by adsorption separation from mixed gases, for example, gases composed mainly of nitrogen and oxygen, as well as to a gas separation method employing it. The gas separation method of the invention is, specifically, a gas separation method based on Pressure Swing Adsorption (hereunder abbreviated to PSA), and gases that may be separated and recovered thereby include oxygen gas, nitrogen gas, carbon dioxide gas, hydrogen gas and carbon monoxide gas.
Of these, oxygen gas is one of the particularly important industrial gases, which is widely used especially for ironworks and pulp bleaching. Recently, oxygen enriched combustion is being accomplished in the field of waste combustion and glass melting for the purpose of reducing NOx emissions that are unavoidable with combustion in air, and therefore oxygen gas is increasing in importance from the standpoint of environmental problems as well.
Known industrial production processes for oxygen gas include the PSA method, the cryogenic separation method, the membrane separation method, etc., but use of the PSA method is increasing because of its advantages in terms of oxygen gas purity and cost.
Oxygen gas production by the PSA method involves selective adsorption of nitrogen gas in the air onto an adsorbent, extraction of the remaining concentrated oxygen gas and collection thereof as the product. The adsorbent used for this purpose is crystalline zeolite which has a large nitrogen adsorption capacity, and particularly X-type zeolite which has a large porous capacity in the crystals is most widely used as the adsorbent for air separation by the PSA method.
Production of nitrogen gas is also possible by utilizing the selectively adsorbed nitrogen gas.
2. Description of the Related Art
X-type zeolite, like Y-type zeolite, is synthetic zeolite in which the crystalline structure is a faujasite structure; such crystals with a relatively low SiO2/Al2O3 molar ratio, i.e. an SiO2/Al2O3 molar ratio of 3.0 or lower, are referred to as X-type zeolite. The SiO2/Al2O3 molar ratio of synthesized X-type zeolite is generally 2.5, but if NaOH and KOH are added during synthesis it is possible to reduce the SiO2/Al2O3 molar ratio to 2.0. Reducing the SiO2/Al2O3 molar ratio of zeolite increases the number of aluminum atoms in the crystals, and therefore the number of exchangeable cations increases. Adsorption of molecules of nitrogen and oxygen onto zeolite is generally known as physical adsorption, and a larger number of exchangeable cations offers a greater adsorption capacity.
Hereunder, X-type zeolite with a SiO2/Al2O3 molar ratio of lower than 2.5, for example, X-type zeolite with a SiO2/Al2O3 molar ratio of from 1.9 to 2.1 inclusive, will be referred to as xe2x80x9clow silica X-type zeolitexe2x80x9d. Processes for production of low silica X-type zeolite are described in Japanese Unexamined Patent Publications (Kokai) (JP-A-53-8400, JP-A-61-222919, JP-A-01-56112, JP-A-10-310422, JP-A-11-217212, and elsewhere).
For industrial use of X-type zeolite as an adsorbent, clay or the like is usually added as a binder to synthesized X-type zeolite powder, and the mixture shaped into pellets or beads. The amount of clay added is about 20-30 parts, and the adsorption capacity of the shaped zeolite decreases by the amount of binder added with respect to the adsorption capacity of the zeolite powder. In order to overcome this, there have been proposed to date production processes for binderless shaped products, which are shaped with almost no binder. Such low silica X-type zeolite shaped products are described in Japanese Unexamined Patent Publications (JP-A-61-222919, JP-A-5-163015, JP-A-11-076810 and elsewhere).
Japanese Unexamined Patent Publication JP-A-61-222919 describes a process for production of a low silica X-type zeolite shaped product, called a macroscopic monolithic body of self-bonding zeolite, whereby no low silica X-type zeolite powder is used, but rather a shaped product of a kaolin starting material is transformed to metakaolin and then crystallized. According to this process, obtaining low silica X-type zeolite requires adding a large amount of a pore-forming substance (organic) to the shaped kaolin, heating and burning to make a porous metakaolin shaped product, and then crystallizing it.
However, because this process is accompanied by a very large exotherm due to burning of the organic substance, the temperature control is troublesome and it is a very difficult matter to successfully control the pores of the shaped product; moreover, since the pores must be actively formed, this creates the problems of notably impaired crush resistance and attrition resistance of the resulting low silica X-type zeolite shaped product. It is also inadequate in terms of the purity of the low silica X-type zeolite during shaping, and for example, A-type zeolite impurities are sometimes included during shaping, resulting in a low concentration of low silica X-type zeolite.
Conventional low silica X-type zeolite has peak intensities at index 111, 220, 331, 533, 642 and 751+555 in the following order.
The macroscopic monolithic body of self-bonding zeolite according to this patent has the same peak intensities in the following order.
In Japanese Unexamined Patent Publication JP-A-5-163015 there is described a process for production of a low silica X-type zeolite binderless shaped product 25 wherein a shaped product comprising X-type zeolite powder with an SiO2 2/Al203 molar ratio smaller than 2.5, kaolin clay transformed to metakaolin, sodium hydroxide and potassium hydroxide, is kept in an aqueous solution of sodium hydroxide and potassium hydroxide at a temperature of 40-100xc2x0 C. for a few hours to a few days for aging and crystallization.
This process requires admixture of dangerous caustic chemicals during the mixing, kneading and shaping, and workability is poor, while the low silica X-type zeolite binderless shaped product obtained by the process naturally has low strength.
Japanese Unexamined Patent Publication JP-A-11-076810 also describes a low silica X-type zeolite shaped product of which at least 95% has an SiO2/Al2O3 molar ratio of 2. The production process is a process in which a mixture obtained by aggregation of low silica X-type zeolite powder with a binder comprising at least 80% clay classified as kaolinite, halloysite, nacrite or dickite which is transformable to zeolite and 15% of montmorillonite as another clay, is shaped and dried and then calcinated at a temperature of 500-700xc2x0 C., after which the resulting product is contacted for a few hours at 95xc2x0 C. with at least a 0.5 molar concentration of a caustic solution, which is a solution of sodium hydroxide and potassium hydroxide, wherein the maximum potassium hydroxide content with respect to the total of sodium hydroxide+potassium hydroxide is 30 mole percent, and specifically with the caustic solution at 5.5 moles/liter. The low silica X-type zeolite binderless shaped product obtained by this process has, unsurprisingly, very low crush resistance and attrition resistance and includes A-type zeolite; moreover, since the SiO2/Al2O3 molar ratio of the total based on chemical analysis or the SiO2/Al2O3 molar ratio of the crystal lattice based on Si-NMR is higher than the theoretically ideal value of 2.0 for low silica X-type zeolite, and particularly the SiO2/Al2O3 molar ratio of the total by chemical analysis is as high as 2.08, the low silica X-type zeolite purity is also insufficient during shaping.
When the crush resistance and attrition resistance of the low silica X-type zeolite binderless shaped product are weak, its use as an adsorbent, for example, when separating and concentrating oxygen from mixed gas composed mainly of nitrogen and oxygen by adsorption separation, where the mixed gas and the low silica X-type zeolite binderless shaped product are repeatedly contacted, results in fracture, flaking or other defects in the shaped product, which can lead to troubles including clogging of the channels and valves of the adsorbent system, increased pressure drop of the shaped product-packed layer, or inclusion of foreign substances in the produced gas.
Moreover, a low purity of the low silica X-type zeolite in the shaped product results in poor adsorption properties, and since A-type zeolite has a particularly high adsorption capacity for oxygen, inclusion of A-type zeolite adversely affects the adsorption properties especially when separating and concentrating oxygen by adsorption separation from mixed gases composed mainly of nitrogen and oxygen.
The present invention avoids the difficulties described above by providing, in a simple, rapid and efficient manner, a high purity, low silica X-type zeolite binderless shaped product with a high content of low silica X-type zeolite, very high crush resistance and attrition resistance, and excellent adsorption performance, as well as a gas separation method whereby nitrogen is efficiently adsorbed and separated from a mixed gas containing nitrogen and oxygen utilizing the high purity, low silica X-type zeolite binderless shaped product as the adsorbent.
The present inventors have achieved the present invention based on diligent research on various factors governing the properties of low silica X-type zeolite binderless shaped products and their production.
The gist of the present invention is as follows.
(1) A high purity, low silica X-type zeolite binderless shaped product, wherein the peak intensity of the faujasite zeolite at the index of 220 is stronger than the peak intensity at the index of 311 according to X-ray diffraction, the content of the low silica X-type zeolite is at least 95% and the SiO2/Al2O3 molar ratio is 2.00xc2x10.05.
(2) A high purity, low silica X-type zeolite binderless shaped product, which is a high purity, low silica X-type zeolite binderless shaped product according to (1) wherein the peak intensities at index 111, 751+555, 642, 533, 220, 311 and 331 for the faujasite zeolite according to X-ray diffraction are in the order listed below, from approximately 60% to approximately 90% of the exchangeable cation sites are sodium, and all or a portion of the remainder are potassium.
(3) An ion-exchanged high purity, low silica X-type zeolite binderless shaped product, which is a high purity, low silica X-type zeolite binderless shaped product according to (1) or (2) wherein all or a portion of the exchangeable cation sites are ion-exchanged with lithium, and when cations other than lithium are present they are cations selected from among alkali metals, alkaline earth metals and transition metals or their mixtures.
(4) An ion-exchanged high purity, low silica X-type zeolite binderless shaped product according to (3), wherein the peak intensities at index 111, 642, 331, 533, 751+555, 220 and 311 for the faujasite zeolite according to X-ray diffraction are in the order listed below.
(5) A high purity, low silica X-type zeolite binderless shaped product or ion-exchanged high purity, low silica X-type zeolite binderless shaped product according to any one of (1) to (4), characterized in that the ratio of the peak intensity attributed to Si-3Al and the peak intensity attributed to Si-4Al according to Si-NMR measurement is such that:
(peak intensity for Si-3Al)/peak intensity for Si-4Al) less than 0.1
(6) A high purity, low silica X-type zeolite binderless shaped product or ion-exchanged high purity, low silica X-type zeolite binderless shaped product according to (5), characterized in that the content of the low silica X-type zeolite is 98% or greater.
(7) A high purity, low silica X-type zeolite binderless shaped product or ion-exchanged high purity, low silica X-type zeolite binderless shaped product according to (5) or (6), characterized in that the average value for the crush resistance measured for representative particles sorted to a particle size of 1.4-1.7 mm is 0.7 kgf or greater.
(8) A high purity, low silica X-type zeolite binderless shaped product or ion-exchanged high purity, low silica X-type zeolite binderless shaped product according to (5) or (6), characterized in that the average value for the crush resistance measured for representative particles sorted to a particle size of 1.4-1.7 mm is 1.0 kgf or greater.
(9) A process for production of a high purity, low silica X-type zeolite binderless shaped product or ion-exchanged high purity, low silica X-type zeolite binderless shaped product according to any one of (1) to (8), characterized by mixing, kneading, shaping and calcining low silica X-type zeolite with an SiO2/Al2O3 molar ratio of from 1.9 to 2.1 inclusive and kaolin clay with an SiO2/Al2O3 molar ratio of from 1.9 to 2.1 inclusive, to obtain a low silica X-type zeolite-containing shaped product, and contacting the low silica X-type zeolite-containing shaped product with a caustic solution to transform all or a portion of the kaolin clay in the low silica X-type zeolite-containing shaped product to low silica X-type zeolite, thereby forming a low silica X-type zeolite binderless shaped product with a SiO2/Al2O3 molar ratio which is lower than the SiO2/Al2O3 molar ratio of the low silica X-type zeolite starting material.
(10) A process for production of a high purity, low silica X-type zeolite binderless shaped product or ion-exchanged high purity, low silica X-type zeolite binderless shaped product according to (9), characterized in that the caustic solution used dissolves a greater amount of Si than Al from the low silica X-type zeolite-containing shaped product.
(11) A process for production of a high purity, low silica X-type zeolite binderless shaped product or ion-exchanged high purity, low silica X-type zeolite binderless shaped product according to (9) or (10), characterized in that the shaped product is contacted for at least 10 hours with a caustic solution of 6 moles/liter or greater.
(12) A process for production of a high purity, low silica X-type zeolite binderless shaped product or ion-exchanged high purity, low silica X-type zeolite binderless shaped product according to any one of (9) to (11), characterized in that the shaped product is contacted for at least 5 hours with a caustic solution of 8 moles/liter or greater.
(13) A process for production of a high purity, low silica X-type zeolite binderless shaped product or ion-exchanged high purity, low silica X-type zeolite binderless shaped product according to (9) or (10) characterized in that the shaped product is contacted with a caustic solution to which Al has been previously added.
(14) A gas separation method characterized by contacting a mixed gas with a packed layer which is packed with one or a plurality of high purity, low silica X-type zeolite binderless shaped products or ion-exchanged high purity, low silica X-type zeolite binderless shaped products according to any one of (1) to (8) and has a combined or multilayer structure, and selectively adsorbing at least one constituent gas of the constituent gases in the gas.
(15) A nitrogen gas/oxygen gas separation method, which is a gas separation method according to (14) characterized in that the gas is air, nitrogen gas is selectively adsorbed onto the packed layer and oxygen gas is allowed to pass through the packed layer for separation from the nitrogen gas.
(16) A nitrogen gas/oxygen gas separation method, which is a nitrogen gas/oxygen gas separation method according to (15) characterized in that in pressure swing adsorption, after selective adsorption of nitrogen gas in the air onto the packed layer under high pressure conditions, the pressure is reduced to desorb the adsorbed nitrogen and restore the packed layer.
(17) A nitrogen gas/oxygen gas separation method, which is a nitrogen gas/oxygen gas separation method according to (16) characterized in that the operation involves an adsorption step of contacting air with the packed layer to selectively adsorb nitrogen and to collect or discharge the concentrated oxygen from an outlet of the packed layer, a regeneration step of interrupting contact between the air and the packed layer to reduce the pressure in the packed layer and to desorb and collect or discharge the adsorbed nitrogen, and a repressurization step of pressurizing the packed layer by the concentrated oxygen obtained in the adsorption step.
(18) A nitrogen gas/oxygen gas separation method, which is a nitrogen gas/oxygen gas separation method according to (17) characterized in that the adsorption pressure during the adsorption step is in the range of 600 Torr to 1520 Torr inclusive.
(19) A nitrogen gas/oxygen gas separation method, which is a nitrogen gas/oxygen gas separation method according to (17) or (18) characterized in that the regeneration pressure during the regeneration step is in the range of 100 Torr to 400 Torr inclusive.
(20) A nitrogen gas/oxygen gas separation method, which is a nitrogen gas/oxygen gas separation method according to any one of (17) to (19) characterized in that the repressurization pressure during the repressurization step is in the range of 400 Torr to 800 Torr inclusive.